One of the small number of monsters that Dungeons & Dragons has spawned completely on its own and made iconic in gaming culture. Like its fellow, the infamous Owlbear, the Rust Monster is a creature created by Gary Gygax, fuelled by inspiration taken from a cheap Chinese plastic figurine he somehow had.
Rust Monsters are roughly dog-sized invertebrates, something vaguely like feathery-antennae'd rust-colored silverfish. They feed on metal by decaying it into rust with a touch, and though they have little interest in flesh, they eagerly attack adventurers due to their tendency to carry large amounts of magical metal. They inspire fear in fighters and other melee classes normally only felt by wizards dealing with anti-magic creatures, since they can instantly destroy precious magical weapons and magic armor and other valuable loot with ease.
Bizarrely, they are also friendly, lovable creatures, and they make cute pets if tamed and kept well-fed with sufficient quantities of typical shitty-monster-quality weapons and armor that many adventurers naturally accumulate as loot. 4E encouraged this by making magic items they destroy leave behind magical stuff worth more than selling the item directly would bring. Drawing them as cuter than they seem is popular.